The present invention relates to a high frequency output driver circuit having a programmable drive capability that does not compromise noise performance of the driver circuit at lower frequencies.
Output drivers integrated as part of a larger circuit often act as a source of noise through power supply coupling. The magnitude of this noise is directly related to a frequency of operation of the driver circuit. To be able to drive capacitive loads at high frequencies, output driver circuit design includes providing an output signal having fast rise and fall times. Achieving fast rise and fall times requires devices with very large channel width to length (W/L) ratios capable of sinking and sourcing large amounts of current. As the rise and fall times of the output waveforms of the output driver become faster, the output driver circuit introduces increasingly higher level harmonics into the circuit through the power supply pins, bond wire and lead frame inductance. For operation at lower frequencies, the output waveform has more time to reach a final value, allowing use of smaller and slower devices handling lower amounts of current. This in turn results in reduced noise levels for the output driver circuit and reduced noise coupling for the larger circuit.
Therefore in applications with wide frequency ranges and high frequency requirements, designers have either compromised the noise performance at lower frequencies to obtain high frequency operation, or compromised the frequency performance at higher frequencies to obtain lower noise levels during the low frequency operation.